LMH6601_09 Datasheet, PDF (25/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

TRANSIMPEDANCE AMPLIFIER NOISE
CONSIDERATIONS
When analyzing the noise at the output of the I-V converter,
it is important to note that the various noise sources (i.e. op
amp noise voltage, feedback resistor thermal noise, input
noise current, photodiode noise current) do not all operate
over the same frequency band. Therefore, when the noise at
the output is calculated, this should be taken into account.
The op amp noise voltage will be gained up in the region be-
tween the noise gain’s “zero” and its “pole” (f_z and f_p in Figure
14). The higher the values of R_F and C_IN, the sooner the noise
gain peaking starts and therefore its contribution to the total
output noise would be larger. It is obvious to note that it is
advantageous to minimize C_IN (e.g. by proper choice of op
amp, by applying a reverse bias across the diode at the ex-
pense of excess dark current and noise). However, most low
noise op amps have a higher input capacitance compared to
ordinary op amps. This is due to the low noise op amp’s larger
input stage.
OTHER APPLICATIONS
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FIGURE 15. Charge Preamplifier Taking Advantage of
LMH6601’s Femto-Ampere Range Input Bias Current
CAPACITIVE LOAD
The LMH6601 can drive a capacitive load of up to 1000 pF
with correct isolation and compensation. Figure 16 illustrates
the in-loop compensation technique to drive a large capacitive
load.
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FIGURE 16. In-Loop Compensation Circuit for Driving a
Heavy Capacitive Load
When driving a high capacitive load, an isolation resistor
(R_S) should be connected in series between the op amp out-
put and the capacitive load to provide isolation and to avoid
oscillations. A small value capacitor (C_F) is inserted between
the op amp output and the inverting input as shown such that
this capacitor becomes the dominant feedback path at higher
frequency. Together these components allow heavy capaci-
tive loading while keeping the loop stable.
There are few factors which affect the driving capability of the
op amp:
• Op amp internal architecture
• Closed loop gain and output capacitor loading
Table 4 shows the measured step response for various values
of load capacitors (C_L), series resistor (R_S) and feedback re-
sistor (C_F) with gain of +2 (R_F = R_G = 604Ω) and R_L = 2 kΩ:
TABLE 4. LMH6601 Step Response Summary for the
Circuit of Figure 16
C_L
R_S
C_F
t_rise/ t_fall Overshoot
(pF)
(Ω)
(pF)
(ns)
(%)
10
0
1
6*
8
50
0
1
7*
6
110
47
1
10
16
300
6
10
12
20
500
80
10
33
10
910
192
10
65
10
* Response limited by input step generator rise time of 5 ns
Figure 17 shows the increase in rise/fall time (bandwidth de-
crease) at V_OUT with larger capacitive loads, illustrating the
trade-off between the two:
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